1. Field of the Invention
This invention relates to a gate drive circuit for a voltage-driven type power switching device which controls the switching on and off of the voltage-driven type power switching device.
2. Description of the Related Art
As a voltage-driven type power switching device, an insulated gate bipolar transistor (hereafter, abbreviated as IGBT) or a field effect transistor is used. Each of these devices has a large gate input impedance, and on-off switching control thereof can be effected by applying a gate voltage to a gate thereof.
A conventional gate drive circuit for an IGBT is shown in FIG. 6. In FIG. 6, a gate voltage v.sub.G output from a control device 2 for controlling the on/off switching of an IGBT 1 is supplied between the gate and emitter of IGBT 1 by means of a signal wire 3, such as a twisted pair wire or the like.
In this case, the wiring is made such that the effect of the magnetic flux due to the main circuit current flowing in the conductor connected to the collector or emitter of IGBT 1 is made as small as possible.
However, in reality, it is not possible completely to eliminate the effect of the magnetic flux resulted from the main circuit current due to the relative physical positioning. Therefore, there are cases where an induced voltage due to this magnetic flux is produced in signal wire 3, which has an adverse effect on a voltage v.sub.GE between the gate and emitter of IGBT 1.
For example, an induced voltage v.sub.X is produced in one part of signal wire 3 by change in a magnetic flux .PHI. due to change in an emitter current i.sub.E of IGBT 1. There are cases where the polarity of this induced voltage v.sub.X is in the direction, as shown in FIG. 7 whereby voltage v.sub.GE applied between the gate and eimtter of IGBT 1 rises or falls when emitter current i.sub.E changes in the direction of an increase or a decrease, respectively. In this case, the following problems arise.
Specifically, when the main circuit current flowing in the collector and emitter of IGBT 1 changes in the direction of an increase, if voltage v.sub.GE between the gate and emitter of IGBT 1 rises, a voltage v.sub.CE between the collector and emitter of IGBT 1 is caused to fall, and thereby a positive feedback action is produced causing the main circuit current to further rise. Therefore, there is the danger that when there is a sudden increase in the main circuit current in IGBT 1 due to shorting of the load, fop example, voltage v.sub.GE between the gate and emitter rises, to thereby cause the main circuit current to rise further, and as a result, IGBT 1 cannot be protected from an overcurrent and may be damaged.
Furthermore, when the main circuit current flowing in IGBT 1 changes in the direction of a decrease, if voltage v.sub.GE between the gate and emitter falls, voltage v.sub.CE between the collector and emitter is caused to rise, and thereby a positive feedback effect is produced causing the main circuit current to further fall. Therefore, there is the danger that when gate voltage v.sub.G output from control device 2 is reduced at a prescribed rate of decrease and the main circuit current in IGBT 1 is reduced to zero, the main circuit current is caused further to fall suddenly due to the positive feedback action, and an excess voltage between the collector and emitter of IGBT 1 is produced by a back e.m.f. resulted from a floating inductance (not shown in drawing) present in the main circuit, and this may cause damage to IGBT 1.
Furthermore, even if IGBT 1 is not damaged by the over current or overvoltage as described above, there is the risk that high-frequency oscillations may be produced in the main circuit current in IGBT 1 due to the aforementioned positive feedback action, which may adversely affect the life etc. of IGBT 1.